Lignin valorization and cleavage of arylether bonds in chemical processing of wood: a mini-review

Lignin valorization is strongly dependent on a right strategy for converting lignin into value-added products. Conversion of lignin into monomeric degradation products is one of the important avenues. In this study, chemical mechanisms and monomeric product compositions in hydrolysis (acidic and alkaline), hydrogenolysis, catalytic oxidation, electrochemical oxidation and reduction, photochemical and enzymatic degradation of native and technical lignins, and lignin model compounds are comparatively analyzed. The effect of the structure of a phenylpropane unit in lignin on the chemical reactivity of α-O-4 and β-O-4 bonds in cleavage reactions is also described. Published experimental data suggest some form of activation to be a necessary prerequisite for the splitting of a β-O-4 bond in all chemical reactions under consideration, the nature of which is dependent on the reagents and reaction conditions. Thus, in catalytic oxidation processes, a benzyl hydroxyl group is converted into carbonyl group at the first stage. Chemical transformation involving the α-position in a phenylpropane unit is a usual trigger of further lignin depolymerization. The yield of monomeric products of hydrogenolysis of native lignin is close to the theoretical one, reaching 23% in softwood and 51% in hardwood; in alkaline hydrolysis as well as oxygen and nitrobenzene oxidation of native lignin, the trend is the same that is explained in this study. On the other hand, the yield of monomeric products from isolated samples and technical lignins is much lower. A loss of arylether bonds in the process of lignin separation from wood and in wood pulping explains this difference.